Air filter



FIG. 1 is a perspective view showing the new design for an air filter;

FIG. 2 is a top view thereof;

FIG. 3 is a bottom view thereof;

FIG. 4 is a front view thereof; and,

FIG. 5 is a side view thereof.

The portions in the drawing views shown in broken lines form no part of the claimed design. In addition, the dot-dash-dot type broken lines shown in the drawing views define the boundaries of the claimed design and form no part of the claimed design. 

CLAIM The ornamental design for an air filter, as shown and described. 